1. Field of the Invention
The present invention relates to a multilayered optical thin-film filter with improved transmission phase characteristics for an optical band pass filter, particularly to an optical filter for use in wavelength division multiplexing (WDM) photonic networks (optical fiber communications networks). The present invention relates to a method of designing such an optical filter, and also to an optical filter module including such an optical filter.
2. Prior Art
In wavelength division multiplexing (WDM) photonic networks, there have been used optical add/drop multiplexers (ADM) for multiplexing a number of wavelength light signals in to an transmission optical fiber and for separating the summed signals into individual wavelengths, the function of which is implemented by use of an optical filter for wavelength segmentation.
As an example of such filters which have conventionally been used, FIG. 21 shows a multilayered optical thin-film filter which is a band-pass filter structured by laminating repeatedly sets of a reflector 97, a spacer 99, and a reflector 98 which together are composed of multiple of optical thin films such that the filter may pass only specific wavelength component light during the signal laser light passing through the multiple layers.
Although the above optical filter can be provided in the form of a relatively simple, thermally stable structure at relatively low cost, a method of designing filters with complex characteristics has not yet been established. Conventionally, such a filter is shown in FIG. 21 which has been produced by empirically finding out a layer structure such that magnitude transmission characteristics are flattened within a pass band including further stacking some cavities with reflectors 97 and 98 via a spacer 99 to sharpen the selectivity.
Generally, the filter with such simple characteristics has a layer structure symmetric to a central plane along the optical axis thereof. However, only terminal layers, which comes into contact with a substrate such as glass plate for mounting the filter, have, breaking the symmetry, a different layer arrangement or different optical constants to create due optical consistency with the substrate.
Conventionally, multilayered optical thin-film filters themselves have complicated and it is difficult to design and produce a filter considering magnitude and phase group delay time characteristics in the filter. Also no method for designing filter performance has been known.
The transmission capacity of today""s photonic networks has been larger, and large-capacity networks such as D-WDM are put into practice. Though the conventional filers have satisfied only the wide selectivity, D-WDM systems requires optical filters having sharper wavelength selectivity in a narrower pass band with lower linear distortion. Although attempts have been made to achieve narrower band filtering characteristics, the filters have not been considered to reduce linear distortions. Although conventional filters satisfy wavelength selective characteristics for effective constraint of disturbing signals in the stop bands, large linear distortion has been retained unimproved which deteriorates signal quality.
Particularly, to severe applications for selecting optical signals which are modulated by high speed and signal wavelength spectra and occupy the entire pass band of the filter, dispersion characteristics of the filter cause a required signal to be subjected to a large waveform distortion, with a demodulated signal deteriorating in quality.
For more complete dispersion constraints, it is necessary to control simultaneously the group delay time characteristics and the magnitude characteristics.
It is an object of the present invention to provide an optical filter with multiple thin layer structure having low linear distortion and high selectivity characteristics.
It is still another object of the present invention to provide a designing method for designing multiple thin-film structure for creating the signal transmission characteristics to be given to the filter.
It is a further object of the present invention to provide an add/drop filter module integrated with the optical filter that is simple in structure and stable in use conditions of heat and the like.
The multilayered optical thin-film filter of the present invention is composed of multiple optical thin films having a thickness of a quarter wavelength xcex0 at a passband center and having different two refractive indices. The multilayered optical thin-film filter includes a plurality of unit cavities which are multilayered through connection layers, each connection layer being disposed between and in contact to the adjacent unit cavities. Each of the unit cavities includes a first reflector layer, a spacer layer in contact with the first reflector layer, and a second reflector layer in contact with the spacer layer. In the-unit cavity, the first reflector layer is a layered body formed by layering a plurality of first two-layer unit films each consisting of an optical thin film having a high refractive index and an optical thin film having a low refractive index in this order, while the second reflector layer is a layered body formed by layering a plurality of second two-layer unit films each consisting of an optical thin film having the low refractive index and an optical thin film having the high refractive index which are arrayed in this order, and the spacer layer is composed of a multiple layer layering an even number of optical thin films having the high refractive index, or is composed of a multiple layer laying an even number of optical thin films having the low refractive index accompanied with an odd number of optical thin films having the high refractive index on the both sides. Each of the connection layers is composed of a single or multiple layer layering an odd number of optical thin films having the low refractive index.
The present invention includes a method of designing the above multilayered optical thin-film filter. In the designing method of the multilayered optical thin-film filter according to the present invention, the layered optical filter structure is determined by defining transmission and reflection characteristics required for the optical filer by a Hurwitz polynomial in a LC filter circuitry theory, expanding the Hurwitz polynomial to a particular prototype low pass filter for electric circuits, and further conducting a frequency transform and equivalent transformations of the prototype low pass filter to the optical filter.
According to the present invention, there may be designed an optical filter with desired band pass filter characteristics, less linear distortion to a passing signal. This eliminates disturbing signals while suppressing linear distortion produced in a desired signal, thereby implementing a multilayered optical thin-film filter capable of maintaining excellent communications quality.
The present invention further includes an optical filter module with a multilayered optical thin-film filter connected in parallel with a phase shifter to an input port via a optical coupler, and the multilayered optical thin-film filter and the phase shifter are connected to an output port via another optical coupler. In this filter, an electrical length of a phase shifter portion including the couplers may be substantially different from an electrical length of transmission path portions before and after the multilayered optical thin-film filter also including the couplers by an odd integer multiplied by a quarter wavelength xcex0 at the passband center.
The optical filter module of the present invention has advantages that the magnitude characteristics and the group delay in the passband may be independently controlled, linear distortions of the signal light caused when it is passing through the filter may be restrained to a lower level, and an attenuation pole may be provided in the stop band for creating larger selective characteristics. Further, the optical filter module of the present invention has advantages that there may be obtained a filter with ideal characteristics that has high selectivity and gives less linear distortion to a passing signal regardless of the high selectivity, as a result of which disturbing signals are eliminated while the desired signal is maintained with linear distortions suppressed. Such multilayered optical thin-film filter module may be incorporated in an add/drop system.